1. Field of the Invention
This invention relates to a working vehicle having a body frame, front wheels, propelling drive shafts mounted in a rear portion of the body frame, and semicrawler units driven by the propelling drive shafts.
2. Description of the Related Art
Such a working vehicle has high utility; it has a wide range of use including operations in paddy fields and other farming fields and on dairy farms. Working vehicles, particularly agricultural working vehicles, may be classified by the running device broadly into the wheel type and crawler type. The wheel type having two axles and four wheels may be subdivided into the four wheel drive type, rear wheel drive type and front wheel drive type. The crawler type may be subdivided into the full crawler type and semicrawler type. (See Agricultural Machine Handbook, New Edition, pages 257-259).
The wheel type working vehicle equipped with a three-point linkage, a PTO shaft and hydraulic control devices may be used in a wide range of operations including plowing, sowing, tilling, weeding, harvesting and transport. This type of working vehicle is used as a power vehicle in all seasons and is a mainstream of agricultural automation.
The full crawler type working vehicle has a large ground-engaging area, and therefore has a smaller ground contact pressure than the wheel type working vehicle. Thus, the full crawler vehicle can run on soft or unleveled grounds. This type of vehicle, with a heavy weight and a large adhesion coefficient, is suited for a heavy operation requiring a strong tractive force. This type of vehicle is a seasonal dedicated vehicle used exclusively in land development or land improvement. Such an operation is impossible with a wheel type working vehicle, and requires a sub-soiler to secure drainage in wet fields, for example.
The semicrawler type working vehicle has belt units in place of rear wheels to act as an apparatus supplemental to the wheel type working vehicle. On soft grounds, the semicrawler working vehicle exhibits a performance intermediate between the wheel type and the full crawler type.
The wheel type working vehicle is highly mobile to be suitable for a wide range of operations. Its disadvantage is that the running performance is low unless the fields become dry in plowing times, e.g. under changeable weather of autumn and the influence of snow in early spring (the latter being called spring plowing practiced substantially during a fixed period in spring in view of the growth and harvest of crops).
In such a situation, i.e. under unfavorable conditions or in very wet fields, the full crawler working vehicle demonstrates an excellent performance. However, this vehicle, which lacks in mobility and is a specialized machine applicable only to a limited operation. When used in a harvesting operation in a dry field, the vehicle would damage the field. After all, both the wheeled working vehicle and the full crawler working vehicle must be made available. This requires a huge investment in agricultural management.
Under such circumstances, semicrawler working vehicles have been proposed as disclosed in Japanese Patent Laying-Open Publication H4-257778 (hereinafter called conventional example 1), and in Japanese Utility Model Publication H6-5974 and Utility Model Laying-Open Publication S63-70488 (hereinafter called conventional example 2).
Conventional example 1 includes rollers disposed rearwardly of the rear wheels, and crawler belts (endless tracks) wound around the rear wheels and rollers. The running devices (rollers) project to a large extent rearwardly of the vehicle body to be obstructive to a working implement using a three-point linkage. Further, the vehicle has an excessively large wheelbase to impair turning performance. A high operating quality cannot be expected when running on unleveled grounds or the like.
Conventional example 2 includes oscillatable semicrawler units to have some adaptability to operations in rugged fields, for example. However, this example is applied to a specialized machine such as a combine harvester. This working vehicle cannot be used in varied operations without changing its construction for attaching the semicrawler units to the vehicle body.